Two-Sided Market

What Does Two-Sided Market Mean?
A market in which market makers (or specialists) are required to give both a firm bid and firm ask for each security in which they make a market. In other words, those making the market must be willing to both buy and sell at the prices they quote. 

Also known as a "two-way market".

Investopedia explains Two-Sided Market
People mainly use this term in the context of the Financial Industry Regulatory Authority (FINRA) requirement that Nasdaq market makers give both a firm bid and firm ask for each security in which they make a market. However, this term can also be applied in the bond market. For example, some broker-dealers make two-sided markets on larger, actively traded bonds and rarely make a two-sided market in inactively traded bonds. The theory is that this helps to enhance liquidity and market efficiency.